The present invention is a structure that can be made of small logs to simulate a natural log jam in a stream for environmental enhancement purposes. The structure may be readily assembled on site from nearby materials thereby minimizing or eliminating the need for accessibility of heavy equipment.
For many years debris in streams, such as large logs or log jams, was regarded as a nuisance or even deleterious to stream health. It was common practice to remove this debris to permit unobstructed stream flow for ease of navigation or flood control. This thinking has changed radically in recent years as efforts to enhance fish populations and restore damaged riparian environments have gained major emphasis. Organic debris is now generally recognized as important for maintaining the biotic and abiotic functions of streams, lakes, and ponds. In streams, large woody debris has a major influence on channel form and on sediment transport and deposit patterns. The quantity of large woody debris is highly correlated with the number and spacing of pools that are critical as fish habitat. This favorable debris may be present as individual large logs or down trees or as log jams formed from an accumulation of logs with smaller brush and tree remains. In addition to providing favorable water conditions for aquatic life, this in-stream wood also provides shade and cover for refuge from predators.
An earlier patent of one of the present inventors, U.S. Pat. No. 5,823,710, describes a substitute for natural large woody debris that can be readily manufactured and transported to the site where it will be used. This product has proved to be a very effective tool for managing stream habitat and hydrology. It is formed as a hollow, generally cylindrical or frustroconical structure formed from an even number of small logs held together by wood struts mortised into the logs. The central portion may be ballasted with rocks to increase the weight and stability against movement by high water. This invention replicates the form and function of an individual large log. However, it cannot readily act to create a larger debris structure, such as a log jam, unless by chance it serves as an accumulation point for woody material floating downstream.
The effects of stream debris on aquatic life habitat and stream flow characteristics has recently received significant theoretical study. Abbe, T. B. and D. R. Montgomery, Regulated Rivers Research and Management 12: 201-221 (1996), describe large woody debris jams and their effect on channel hydraulics and habitat formation in large rivers. They note the long term stability over many years of individual natural log jams. The authors describe three general types of log jams and how each affects stream hydraulics.
Slaney, P. A., R. J. Finnegan, and R. G. Millar in Fish Habitat Rehabilitation Procedures, P. A. Slaney and D. Zaldokas eds., Watershed Restoration Technical Circular No. 9, British Columbia Ministry of Environment, Lands and Parks (1997) go into considerable detail as to the types of natural log jams and their effect on stream hydraulics. They describe two basic constructions for anchored man-made log jams. One is a single log type and the other a triangular structure. Both are cabled to bank supporting points and to large boulders introduced adjacent to and within the structures. The structures described emphasize logs buried or partially buried in the stream bank or bottom to act as anchors.
D""Aoust, S. G. and R. G. Millar, Journal of Hydraulic Engineering, November 2000, pp 810-817, describe the retention and stability of natural woody stream debris. They develop mathematical models of stream conditions which affect and are affected by designed ballasted large wood debris structures; e.g., such as those described by Slaney et al.
Castro, J. and R. Sampson, in a draft USDA, Natural Resources and Conservation Service, Portland, Ore., Engineering Technical Note No. 25 (October 2000), give a similar but somewhat less mathematical treatment of the subject dealt with by D""Aoust et al.
A publication by the U.S. Army Corp of Engineers, Green/Duwamish River Basin Ecosysten Restoration Study, King County Washington (2000) illustrates a number of different types of anchored debris and boulder structures for stream flow control. These require logs buried into the shoreline or stream bottom for anchoring.
Most of the systems noted above have complex anchoring systems, require relatively large logs, and/or require medium to heavy equipment for their installation. Additionally they tend to act as dams that direct water around them rather than attenuating water velocity and permitting flow through the structures. The present invention serves the need for a simple, stable, and readily on-site constructed large woody debris structure for stream enhancement.
Engineered log jams have been defined as permanent or semi-permanent structures designed to simulate natural log jams. They contain key pieces of wood which are large enough to affect the course of a river channel. Typically they are also designed to capture additional woody debris. The present invention is a structure serving as a man-made and emplaced log jam for controlling localized stream flow characteristics. It is useful for streamside erosion control, creation of pools for enhancement of habitat for aquatic life, and for rehabilitation of degraded stream locales to a more natural condition. The structures may be readily built and installed in place without the need for heavy equipment and may usually be constructed of locally available materials. They have excellent permanence and are resistant to destruction by floods and other high water events.
The term xe2x80x9clogxe2x80x9d is understood to be a tree stem that will have a diameter of at least about 10 cm (4 in). There is no limit to the maximum diameter and length of the logs but normally they would be of a size to be handleable by two or three persons or by very light equipment, such as a small tractor or backhoe.
The term xe2x80x9cstreamxe2x80x9d should be read broadly to include streams from a few meters across up to large rivers.
The structures are formed by a first layer of at least two spaced apart, generally parallel logs. These are most usually placed against and anchored to the stream bottom, generally parallel to the stream flow. A second layer of logs is then placed across and on top of the first logs. These are attached to the first layer logs at the crossing points and serve to direct stream flow through and over the structure. The second layer also consists of two or more logs spaced apart from each other and angled with respect to each other so that the second layer logs are more widely separated at one end than the other. Projections along the longitudinal axes of the second layer logs will converge at some point beyond the more closely spaced ends. In use, the second layer log at the most upstream location will typically be placed across the stream flow direction. This tends to attenuate flow velocity as water passes over and/or under the log. The downstream log or logs of the second layer will be angled somewhat from the upstream log. This is to take advantage of the effect that regardless of the angle of incidence of water hitting the upstream side of a log, it will leave the log normal to the longitudinal axis. Thus, the logs forming the second layer act to adjust stream flow into a desired direction as it passes over and through the structure.
A third layer of at least two spaced apart logs is then placed so as to cross the logs of the second layer. Again, the second and third layer logs are attached to each other at the crossing points. The logs of this layer cooperate with the second layer logs to further guide water outflow in the desired direction from the structure.
Under most operating conditions, at least the first and second log layers will be underwater although this may vary with normal seasonal changes in stream flow.
It is entirely feasible to add additional log layers above the third layer. In this case, each additional layer will comprise a plurality of spaced apart logs, the logs of each layer crossing and being attached to the logs of the adjacent layers. An exception might be the use of one or more logs or other material placed atop the structure primarily to add dead weight. These may or may not be attached to some or all of the underlying logs.
In most applications the structure will be anchored to a stream bank or stream bottom using cables or other attachment means. It may also be used mid stream; e.g., as a gravel bar apex log jam. Alternatively, it may be used as a floating log jam with anchor points either mid-stream or to some adjacent point on shore. In this location it will normally be about 80% underwater. When installed adjacent a stream bank the more widely separated ends of the second layer logs will usually be oriented mid-stream.
Additional xe2x80x9cjackstrawxe2x80x9d logs may be inserted between the logs forming the main part of the structure. These may or may not be anchored in the stream bottom but will protrude generally vertically from the structure. Their function is to catch floating debris moving downstream and thus enlarge the structure in the manner of a natural log jam.
The structure differs in a number of important ways from those described in the prior art, both in construction and function. The orientation of the first layer logs parallel to stream flow direction greatly simplifies initial installation since it greatly reduces hydraulic resistance and it is not necessary to fight against the current. Additionally, the structure is designed so that water can flow over and through the logs, simulating a natural log jam. This has a number of advantages. Water velocity is attenuated but the water does not meet the resistance of a more solid structure acting as a dam. It also serves as a shelter for aquatic life. Fish can swim over and through the structure yet still find shade and protection from predators. Further, the structure does not require the large boulders that are an integral part of most engineered log jams known to date. These may or may not be available near the site of installation and require fairly heavy equipment to place them because of their extremely high weight.
It is an object of the invention to provide an engineered log jam that may be readily constructed on site with simple equipment.
It is another object to provide an engineered log jam that is effectively designed for hydraulic flow routing and aquatic life habitat formation.
It is a further object to provide an engineered log jam that will resist displacement from peak stream flows.
These and many other objects will become readily apparent to those skilled in the art who will readily understand the construction and operation of the invention by reference to the following detailed description taken in conjunction with the drawings.